Alternate CDS selection function (function code X'04')

The alternate CDS selection function is invoked at a CDS before it forwards a Locate search to any alternate CDSs in the network. An alternate CDS is a central directory server that is used to receive Locate search requests when a search of the database of another CDS fails.

When VTAM® passes the CDS list to the exit, the first entry in the list identifies the CDS with the least weighted path. The remainder of the list continues in least weighted order for all other known CDSs in the network. The exit can approve the list as received, reorder the list, or remove entries from the list. If no entries are returned in the CDS list, VTAM does not forward the Locate request to any alternate CDS and, instead, perform a broadcast search for the resource. If entries are returned in the list, VTAM searches the alternate CDSs in the order specified in the CDS list. The search terminates when a CDS in the list returns information about the resource or all CDSs in the list have been searched.

The following conditions in the returned CDS list are considered not valid and result in VTAM issuing a FFST™ probe, deactivating the function, and continuing with the original list:

Entries are added that are not in the original list.
Duplicate entries are found in the list.
Entries are found that contain corrupted data.

The user-written exit returns to VTAM with a return code:

Register 15:

Return code

X'00' (0): The CDS list has been modified.
X'04' (4): The CDS list has not been modified.

An unexpected return code is treated as return code X'04', but message IST1183I is issued to record the unexpected return code.